1. Field of the Invention
The invention relates to electrographic printing.
In electrographic printing it is usual to have a plurality of exposed styli and to move a recording medium past the exposed styli. The record medium is conventionally a conductive material having a dielectrically coated surface which is presented to the styli as the record medium moves past the head. The head is responsive to input signals and arranged to select individual styli or groups of styli so as to form a surface charge image on the recording medium, which image is later developed elsewhere in the printing system.
2. Description of the Prior Art
In U.S. Pat. No. 3,859,960, selected styli and a backing plate behind the record medium are energised simultaneously with signals of opposite polarity to enable writing. In U.S. Pat. No. 3,611,419, the backing plates are replaced by "front" plates on either side of the stylus array. As in U.S. Pat. No. 3,859,960, the front plates and styli are energised by signals of opposite polarity, on the styli and front plates, and a capacitive coupling is formed with the record medium to enable writing. In both U.S. Pat. No. 3,859,960 and U.S. Pat. No. 3,611,419, it is normally required in practice, to avoid false writing, to energise groups of styli separated by groups of non-energised styli. This is achieved as specifically described in U.S. Pat. No. 3,611,419 by making sure adjacent front plates are not energised simultaneously.
Significant electrical energy is often required to ensure that the required relative potential between styli and record medium is achieved to enable writing by causing ionisation of the gap between the styli and the medium so that the surface charge image can be formed. Ionisation takes place when the potential rises above the "Paschen breakdown voltage".
The "Paschen breakdown voltage" is the voltage at which the insulation of the air breaks down and an avalanche condition ensues allowing ions to flow from the stylus to the record medium. The breakdown voltage varies with the profile of and separation between stylus and record medium and ambient conditions as well as the inherent properties of the record medium.
Normally, the separation of the styli and the record medium is maintained reasonably constant but ambient conditions can vary widely as can the properties of the record medium.